1. Field of the Invention
The present invention relates to an annular fuel rod, and more particularly to a lower and upper end plug of an annular fuel rod, in which a filter for debris is installed in front of the inner channel main inlet of a lower end plug, thereby intercepting an inflow of debris in which at least one inner channel auxiliary inlet with a through-hole shape is formed in a cylindrical wall of said lower end plug, thereby supplying cooling water when the debris is caught up in the debris filter to thus prevent the inner surface temperature of a fuel rod from increasing excessively and in which the upper end plug is provided with a groove in which a tool for pulling the fuel rod is located, thereby making it easy to handle the fuel rod when a nuclear fuel assembly is being assembled or disassembled.
2. Description of the Prior Art
In general, a commercial nuclear power plant is equipped with nuclear fuel assemblies. As illustrated in FIGS. 1 and 2, the nuclear fuel assembly 100 comprises fuel rods 101, guide tubes 103, grid spacers 105, a lower end fitting 106, and an upper end fitting 107.
Each fuel rod 101 is provided with lower and upper end plugs 108 and 109 at lower and upper ends thereof.
Here, each fuel rod 101 has cylindrical uranium pellets inserted into a cladding tube of zirconium alloy. These uranium pellets undergo nuclear fission to generate a high-temperature heat.
Meanwhile, the support grids 105 support the fuel rods 101 and function to maintain a distance between the fuel rods 101. Each guide tube 103 is used as a passage for a control rod that moves up and down in order to control a heat flux of the fuel rods 101.
The upper end fitting 107 and the lower end fitting 106 function to hold and support the nuclear fuel assembly 100 to the upper and lower structure of a reactor core. If necessary, the lower end fitting 106 includes a filter (not shown) for filtering debris floating in the reactor core.
Meanwhile, the lower and upper end plugs 108 and 109 installed at the lower and upper ends of each fuel rod have a conical shape, are welded to the cladding tube (not shown), and prevent the internal gas charged into the cladding tube from leaking out.
Generally, 150 or more nuclear fuel assemblies 100 are loaded into the reactor core.
Meanwhile, the nuclear fuel assembly 100 has cylindrical fuel rods, and is designed so that the cooling water flows through subchannels, each of which are surrounded by four rods or a combination of three rods 101 and one guide tube 103, in an axial direction, or gaps between the rods 101.
At this time, each subchannel 111, surrounded by the rods 101, refers to a channel, the circumference of which is typically partly open to allow fluid to freely move to an adjacent subchannel when used.
As illustrated in FIGS. 3, 4 and 5, there has been disclosed an annular fuel rod 201 that is different from the cylindrical fuel rod has been reported (U.S. Pat. No. 3,928,132 (1975), title: Annular Fuel Element for High-Temperature Reactors, and inventor: Roko Bujas).
Here, each annular fuel rod 201 comprises at least one annular pellet 203, an inner cladding tube 205 provided at an inner circumference of the pellet 203, and an outer cladding tube 207 provided at an outer circumference of a pellet 203.
For this configuration, cooling water flows through the external subchannels 213 and inner channels 211 of the fuel rods 201 at the same time, and absorbs the heat generated from the fuel rods 201. However, an annular fuel rod 201 increases its heat area per bundle to decrease its heat flux, so that it can maintain a low inner surface temperature therein, when compared to a cylindrical fuel rod.
In this manner, in the case in which a low inner surface temperature of each fuel rod 201 is maintained, the possibility of damaging a fuel surface due to an increase in the inner surface temperature is reduced, and thus the safety allowance limit of the fuel rods 201 can be increased.
In the above-described cylindrical fuel rods and annular fuel rods, each cylindrical fuel rod is characterized in that, because the cooling water flows through the subchannels, the cooling water moves freely between the adjacent subchannels. This movement of the cooling water between the adjacent subchannels is based on various factors. The main factor is the loss of pressure generated in the subchannel region, and thus a movement of the cooling water occurs in order to maintain an equilibrium pressure.
Meanwhile, in the case of the annular fuel rod, an external subchannel exchanges cooling water freely with the adjacent external auxiliary channels, because the inner channel makes it impossible for the cooling water to move between the channels because it is enclosed by the inner cladding tube.
Therefore, the debris floating in the cooling water is caught in the inner channel, thereby blocking the inner channel. Further, in a case where the debris blocks the inner channel, it obstructs the flow of the cooling water to reduce the flow rate in the inner channel. For this reason, the heat generated from the wall of the inner channel cannot be sufficiently removed, and thus the temperature of the channel wall surface increases. Furthermore, when the temperature continues to increase, the wall surface of the inner channel is damaged which can cause accidents such as damage to the nuclear reactor. Accordingly, the advantages of the annular fuel rod are decreased.
As described above, in order to prevent the temperature from excessively increasing at the surface wall of the channel, a smooth supply of cooling water to the channel is important when operating a nuclear reactor. To this end, it is important to secure a fluid channel to supply the cooling water, capable not only of minimizing the amount of debris flowing into the inner channel but also cooling the surface of the inner channel even if debris caught in the debris filter blocks the main inlet of the inner channel.
To this end, a nuclear fuel assembly, to which a filter for removing the debris floating in the inner channel is applied, was proposed by Korean Patent No. 10-0074788 (1994), 10-0077453 (1994), 10-0074788 (1994), 10-0010878 (1999), 10-0453268 (2004), and so on. However, the existing debris filter is applied to an existing nuclear fuel assembly with cylindrical fuel rods, and is adapted to be integrally formed with a lower end fitting. For this reason, there is a possibility of debris flowing through a gap between the lower end fitting and the adjacent lower end fitting and through a gap between the lower end fitting and the wall of a reactor core, to thus enter into the subchannels.
Further, the debris filter of the lower end fitting of the nuclear fuel assembly cannot completely filter the debris, and thus the debris flowing from the lower end fitting flows into the subchannel.